The National Institute for Occupational Safety and Health (NIOSH)NIOSH Mining

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Original creation date: August 2004

The roof rock in underground limestone mines in northern Appalachia can be subject to high horizontal stresses despite the shallow depth of the workings. The high stresses can cause roof stability problems. NIOSH researchers have observed a distinctive, asymmetrical mode of failure at the pillar-roof contact in two underground stone mines, which is different from the typical failure mode observed in response to excessive levels of horizontal stress. A dip of greater than 5 deg was identified as a possible cause of this mode of failure. Numerical model experiments showed that an increase in the dip of the workings can cause an increase in the stress at the up-dip corner of the roof beam. A case study is presented in which failure at the pillar-roof contact was observed where the dip of the workings was 7 deg in a high horizontal stress field. Numerical modeling showed that the relatively small dip of the workings could have induced stress changes in the immediate roof that would explain the failures. The model results also showed that this mode of failure is less likely to occur if the limestone pillars contain weak bedding planes that provide stress relief. In addition, the results showed that for the conditions at the case study site, the stress in the roof beam was not sensitive to the thickness of the roof beam or the excavation span. The high horizontal stresses at this site are an important contributing factor to the observed failures.